J.M.
Hi H.,
I have 3 kids (one of which is a new baby) and I was told by my ped. and the fire department that behind the passenger seat is the least safe place in the car, therefore if you have a combo of infants/toddlers in the car, then the INFANT should go in this position. This is because their seat is the most protective (more so than a booster or front facing harness seat). Behind the driver is the next safest spot so the seat with the next highest protection should go there, and the child in the least protective seat (like a backless booster) should go in the middle. It is kind of a weird decision to make, like which one of my kids gets the safest spot, but what they said to me does make sense (esp since I have one infant seat, one front facing harness and one backless booster) so I went with it. I have never had problems with an older sibling bugging the baby (even when I just had two) so depending on the personality of your 3 y/o, you may WANT them to be right next to each other. I have actually found it easier for them to be right next to the baby in case you need to have them start giving them a bottle in a pinch, or hand them a pacifier or toy while driving. Nothing raises the stress level like a crying baby in the back seat when you are driving and feel helpless :) Congrats to you whatever you decide!